Compression ignition engines produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions control devices known as oxidation catalysts (or diesel oxidation catalysts) are commonly used to treat carbon monoxide (CO) and hydrocarbons (HCs), including the volatile organic fraction (VOF) of particulate matter (PM), in exhaust emissions produced by compression ignition engines. Such catalysts treat carbon monoxide (CO) by oxidising it to carbon dioxide (CO2), and treat hydrocarbons (HCs) by oxidising them to water (H2O) and carbon dioxide (CO2).
Oxidation catalysts for compression ignition engines typically comprise platinum group metals (PGMs) and a support material, which have been coated onto a substrate. However, oxidation catalysts take several minutes to heat up to their effective operating temperature from a cold start, and in that time a significant amount of pollutant can be emitted into the air.
One way of bringing the oxidation catalyst up to its effective operating temperature is to incorporate an electrical heater into the exhaust system or the emissions control device. For example, EP 0579415 A1 describes an upstream heater for a catalyst in an exhaust system. The exhaust system contains a catalyst for the treatment of exhaust gas to combat air pollution, and a non-catalytic heater upstream of the catalyst to reduce the time it takes the catalyst to reach its effective reaction temperature. The heater has at least 15 heated channels per square cm, as measured at right angles to the gas flow through them, the heated channels being less than 1.0 cm in length, so that in operation the gas passes through the channels, and is thereby heated before it reaches the catalyst.